Cold rolled steel sheets are widely used in various fields with a focus on uses in automotive bodies. For such uses, they are subjected to press-forming to be made available for use. In particular, from a viewpoint of recent strengthening of CO2 emission regulations, usage ratios of high-strength steel sheets tend to increase with the aim to reduce weights of automotive bodies. However, of cold rolled steel sheets, for those having a base material with a strength exceeding 440 MPa, the higher the strength of the base material, the higher the contact pressure during press forming becomes and the more the hardness of the cold rolled steel sheet approaches the hardness of a mold, and thus, there is a problem in that scratches called mold galling are likely to occur, which are formed by a cold rolled steel sheet being galled by a mold. More specifically, during continuous press forming, molds are severely worn down, and this adversely affects productivity of automobiles, by ruining the appearance of molded articles, for example.
As a method for suppressing the occurrence of mold galling in cold rolled steel sheets, a surface treatment on molds is widely used. In this method, however, adjustment of molds cannot be performed after a surface treatment. Further, this method has another problem of high cost. Therefore, there is a strong demand for cold rolled steel sheets harder to cause mold galling (with improved mold galling resistance).
A large number of techniques have been reported for forming lubricating films on the surfaces of cold rolled steel sheets. Patent Literature 1 describes a technique for forming on a steel sheet a lubricating film containing: an alkali metal borate; and, as a lubricant, a mixture of zinc stearate and a wax.
Patent Literature 2 describes a technique for forming on a steel sheet a lithium silicate as a film component with a wax and a metallic soap, which are added thereto as lubricants.
Patent Literature 3 describes a lubricant-coated steel sheet in which a polyurethane resin having a silanol group or a hydroxy group is formed with a thickness of 1 to 15 μm, and which has excellent continuous formability in high contact-pressure forming.